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{
LR(0) set construction. For an explanation of this algorithm, see
Aho/Sethi/Ullman, "Compilers : Principles, Techniques and Tools,"
1986, Section 4.7.
Copyright (c) 1990-92 Albert Graef <ag@muwiinfa.geschichte.uni-mainz.de>
Copyright (C) 1996 Berend de Boer <berend@pobox.com>
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
$Revision: 1.2 $
$Modtime: 96-07-31 14:09 $
$History: YACCLR0.PAS $
*
* ***************** Version 2 *****************
* User: Berend Date: 96-10-10 Time: 21:16
* Updated in $/Lex and Yacc/tply
* Updated for protected mode, windows and Delphi 1.X and 2.X.
}
unit YaccLR0;
interface
procedure LR0Set;
(* constructs the LR(0) state set, shift and goto transitions and
corresponding kernel items *)
implementation
uses YaccBase, YaccTabl;
(* This implementation is based on the algorithm given in Aho/Sethi/Ullman,
1986, Section 4.7. *)
procedure get_syms ( var item_set : ItemSet; var sym_set : IntSet );
(* get the symbols for which there are transitions in item_set *)
var i : Integer;
begin
with item_set do
begin
empty(sym_set);
for i := 1 to n_items do
with item[i], rule_table^[rule_no]^ do
if pos_no<=rhs_len then
include(sym_set, rhs_sym[pos_no]);
end;
end(*get_syms*);
function make_state ( var item_set : ItemSet; sym : Integer ) : Integer;
(* construct a new state for the transitions in item_set on symbol sym;
returns: the new state number *)
var i : Integer;
begin
with item_set do
begin
(* add the new state: *)
new_state;
for i := 1 to n_items do
with item[i], rule_table^[rule_no]^ do
if (pos_no<=rhs_len) and (rhs_sym[pos_no]=sym) then
add_item(rule_no, pos_no+1);
make_state := add_state;
end;
end(*make_state*);
procedure add_next_links;
(* add links to successor items for kernel items in the active state *)
var k, i : Integer;
begin
with state_table^[act_state] do
for k := trans_lo to trans_hi do
with trans_table^[k] do
for i := item_lo to item_hi do
with item_table^[i], rule_table^[rule_no]^ do
if (pos_no<=rhs_len) and (rhs_sym[pos_no]=sym) then
next := find_item(next_state, rule_no, pos_no+1 );
end(*add_next_links*);
procedure LR0Set;
var act_items : ItemSet;
act_syms : IntSet;
i : Integer;
begin
(* initialize state 0: *)
new_state;
add_item(1, 1); (* augmented start production *)
act_state := add_state;
(* build the state table: *)
repeat
(* compute the closure of the current state: *)
get_item_set(act_state, act_items);
closure(act_items);
(* sort items: *)
sort_item_set(act_items);
(* determine symbols used in shift and goto transitions: *)
get_syms(act_items, act_syms);
(* add transitions: *)
start_trans;
for i := 1 to size(act_syms) do
if act_syms[i]=0 then
(* accept action *)
add_trans(0, 0)
else
(* shift/goto action *)
add_trans(act_syms[i], make_state(act_items, act_syms[i]));
end_trans;
(* add next links to kernel items: *)
add_next_links;
(* switch to next state: *)
inc(act_state);
until act_state=n_states;
end(*LR0Set*);
end(*YaccLR0*).
|
